1. Field of the Invention
The present invention relates to a device for detecting the inclination of a substrate with respect to a predetermined reference plane and more particularly to an automatic leveling device in an exposure apparatus for manufacturing semiconductor devices.
2. Related Background Art
In a reduction projection type exposure apparatus for manufacturing integrated circuits, an object lens having a great numerical aperture (N.A.) is generally used. The depth of focus of the object lens is very small. Therefore, in order to expose the pattern of a mask over an entire exposure region on a wafer clearly, the exposure region on the wafer is required to coincide with an image surface of the object lens. Accordingly, the exposure apparatus is provided with a device for detecting the inclination (horizontal position) of the exposure region on the wafer with respect to a plane perpendicular to the optical axis of the object lens.
Such a device is disclosed in U.S. Pat. No. 4,558,949. The device has an irradiating optical system for emitting a parallel light flux to the substantially whole exposure area (shot area) on the wafer and a condensing optical system for condensing the light flux reflected by the wafer surface on a light receiving element by means of a condensing lens. Upon receiving the reflected light, the light receiving element outputs a detection signal corresponding to its light received position. Based on the detection signal, the horizontal position of the shot area on the wafer is detected.
Also, there is a case that a plurality of IC devices (hereinafter referred to as "chips") are formed in a shot area. Now, a partially broken or incomplete shot area located in a peripheral portion of a wafer is considered. If there are one or more complete chips in the broken shot area, the pattern of a reticle can be exposed repeatedly one over another even on the broken shot area. A device for detecting the horizontal position of such a broken shot area is disclosed in U.S. Pat. No. 4,902,900.
The device is provided with a turret plate having a plurality of field stops whose shapes and dimensions are different from each other in an irradiating optical system for emitting a parallel light flux to a detection area (shot area) on a wafer. Then, a desirable field stop is selected in accordance with the position of a shot area on a wafer and a parallel light flux is emitted to the entire surface of a portion of the shot area wherein there are not broken chips.
However, in the above prior art, when the material of the wafer, the kind of base (metallic film, insulating film, etc.) applied on the wafer, the kind of photoresist applied on the base, etc. are changed to cause the reflectance of the light (the reflectance of the wafer) to be changed, the intensity (quantity) of the reflected light incident on a photoelectric detector is changed. Also, when light having comparatively high coherence is utilized as the light source, the light reflected by the surface of the photoresist and the light reflected by the base are liable to interfere with each other. Accordingly, even though the thickness of the photoresist is changed slightly, the intensity of the reflected light incident on the photoelectric detector is changed largely. Further, the intensity of the reflected light incident on the photoelectric detector is changed due to the structure of the circuit pattern formed on the shot area, too. Furthermore, the intensity of the reflected light entering the photoelectric detector is changed due to changes of shape and size of the illumination area of the parallel light flux.
Thus, when the reflectance of the wafer, the structure of the circuit pattern on the shot area, the shape and size of the illumination area (these are called "reflective characteristics" hereinafter) are changed and the intensity of the light flux incident on the photoelectric detector is lowered, the S/N ratio of a photoelectric signal output from the photoelectric detector is lowered, causing an error in the detection result of the horizontal position.